To meet the miniaturization requirements of airborne RF front ends, a K-band four-channel antenna interface unit was designed based on the System in Package (SiP) technology. A secondary superheterodyne frequency conversion framework was adopted in the proposed unit. Thus, the unit was mainly composed of two silicon-based dual-channel integrated SiP modules and two integrated phase-locked loop SiP modules. The silicon-based frequency conversion SiP module realized the stacking of different functional circuit packages with the Package on Package (PoP) architecture. It ensured good electromagnetic compatibility and achieved output spurious suppression. Moreover, the stacked silicon-based packages were implanted in the ceramic package via BGA to achieve highly reliable airtightness. Based on SiP technology, the volume of the four-channel antenna interface unit was merely 58.2 mm×40.3 mm×11.0 mm. Along with achieving excellent electrical performance, the volume and weight of the proposed unit were significantly reduced by more than 86% compared to a traditional antenna interface unit. Hence, the proposed unit has significant application potential on platforms with rigorous lightweight and miniaturization demands, such as airborne and missile-borne platforms.